Gas turbine engines typically include a compressor, a combustor, and a turbine. The compressor compresses air drawn into the engine and delivers high pressure air to the combustor. In the combustor, fuel is mixed with the high pressure air and the air/fuel mixture is ignited. Products of the combustion reaction in the combustor are directed into the turbine where work is extracted to drive various components of the gas turbine engine, such as the compressor.
In some gas turbine engines, multiple stages of the compressor are driven at a single rotational speed by the turbine. Because each stage of the compressor is not driven at its individual optimal rotational speed by the turbine, these gas turbine engines present operating efficiency limitations. In other gas turbine engines, multiple stages of the compressor are driven by separate turbine units at different rotational speeds. Due to the multiple turbine units used to drive each compressor stage, such gas turbine engines present cost and complexity limitations. In still other gas turbine engines, multiple stages of the compressor are driven by separate turbine units at different rotational speeds, and one of the rotational speeds is dependent upon the rotational speed of the output shaft of the gas turbine engine. Such gas turbine engines present stability problems that may necessitate additional features that increase cost and/or reduce operating efficiency of the gas turbine engines.